In recent years, petroleum and gas resources located on land and in shallow sea areas are being depleted, and deep-sea submarine oil fields have been actively developed. In a deep-sea oil field, crude oil or gas has to be carried from a wellhead set on the sea bottom to a floating platform by use of a flow line or a riser.
A flow line laid in the deep sea that accepts high internal fluid pressure with a deep formation stratum pressure to the inside suffers repeated distortion due to ocean waves and, during an operation stop, deep-sea water pressure. Therefore, steel pipes for the above-mentioned flow line require thick wall stainless pipes with high strength and high toughness, when considering a collapse and metal fatigue, in addition to the strength.
Such a seamless steel pipe with high strength and toughness has previously been manufactured by piercing a billet heated to a high temperature by a piercing mill, rolling and elongating it into a pipe shape product, and then performing a heat treatment. By this manufacturing process, high strength, high toughness and weldability are given to the steel pipe.
In recent years, from the viewpoint of the energy saving and short-cut process, simplification of the manufacturing process has been examined by applying inline heat treatment, that is, a heat treatment in pipe making line. Particularly, paying attention to effective use of the heat of steel after hot-working, a process of quenching a pipe without cooling to room temperature after making in a pipe is introduced, whereby significant energy saving and an increase in efficiency of the manufacturing process can be attained, which effectively reduces the manufacturing cost.
The inline heat treatment process, quenching directly after finish rolling, tends to cause coarse-grained crystal, because the process does not cool the steel pipe to room temperature after rolling, and the steel pipe does not undergo the transformation and reverse transformation process. This results in the difficulty of obtaining good toughness and corrosion resistance.
Therefore, several techniques have been proposed in order to solve this problem. One is a technique for making fine-grained crystal of the finish-rolled steel pipe. Another is a technique that ensures the toughness and corrosion resistance even in a steel pipe having so fine-grained crystal.
For example, the following Patent Document 1 discloses a technique for making the fine-grained crystal after finish rolling, which reduces the steel pipe temperature once to a low temperature (Ac1 transformation point—100° C.) before putting it into the reheating furnace, by adjusting the time from the finish rolling to the putting it into the reheating furnace.
The following Patent Document 2 discloses a technique for manufacturing a steel pipe that has a satisfactory performance even with relatively large grained crystal by adjusting the chemical composition, particularly, the contents of Ti and S.
[Patent Document 1]
Japan Patent Unexamined Publication No. 2001-240913
[Patent Document 2]
Japan Patent Unexamined Publication No. 2000-104117
The recent activated development of large depth submarine oil fields leads to an increase in demand of thick wall steel pipes with high strength. However, it is difficult to provide sufficient performances to the steel pipes by the techniques disclosed in the above patent documents. In thick wall steel pipes that are intended by the present invention, for example, the temperature of finish rolling is increased, and excessive time is needed until the temperature of the steel pipes is down to the required low temperature (Ac1 transformation point—100° C.), thereby the production efficiency is significantly reduced. Therefore, it is difficult to apply the method disclosed in the Patent Document 1 to the thick wall pipes. Furthermore, since the cooling rate of the inline heat treatment for the thick wall pipes is small, the steel having a composition disclosed in the Patent Document 2 also has the problem of deterioration of toughness.